The 511 keV positron annhilation emission from the Galactic bulge has yet to solely be attributed to any astrophysical source of positrons. The 511 keV line, characteristic of electron-positron annihilation, was discovered in 1970 by Johnson and Haymes  almost 40 years ago. Most recently the INTERnational Gamma-Ray Astrophysical Laboratory (INTEGRAL) [Knödlseder 1997] has measured the annihilation rates of both components of the Galaxy. The bulge and disk annihilation rates are (1.5 ± 0.1) × 10 43 e+ s-1 and (0.3 ± 0.2) × 1043 e+ s-1, respectively. The ratio of the bulge rate to that of the disk ranges from 3-9.
Many astrophysical galactic sources have been proposed to be the source of positrons. The proposed sources are massive stars including core collapse supernovae, Wolf-Rayet stars, and gamma ray bursts, cosmic rays, compact objects like X-ray binaries, light dark matter, and thermonuclear supernovae (SNIa). Some sources are known to produce positrons while others, such as light dark matter, are theorized to produce positrons. Many of these sources exist in the bulge and disk of the Milky Way. We examine each source and eliminate many as the source of the Galactic 511 keV positron annihilation emission because they do not exhibit the morphology of the emission. The 511 keV is located at the Galactic center and the emission from the bulge is ∼ 3 times larger than in the disk.
We look at SNIa as the source of the 511 keV emission. SNIa are estimated to have a positron escape yield of 8 × 1052 e + SN-1 and are associated with old stellar populations similar to the Galactic bulge. However, the calculations show that SNIa cannot be the source of the positron annihilation emission because more emission comes from the disk. Although, it is now a fact that SNIa are not "standard candles". It is plausible that supernovae of different luminosities, the SNIa sub-class of subluminous or overluminous, may produce different amounts of positrons and/or have different positron escape yields. We look into subluminous SNIa as the source of positrons and conclude that subluminous supernovae cannot have a positron escape yield to match INTEGRAL observations of 1:5 × 1043 e+ s-1.
|Commitee:||Hartmann, Dieter, King, Jeremy|
|School Location:||United States -- South Carolina|
|Source:||MAI 47/06M, Masters Abstracts International|
|Keywords:||Galactic emission, Gamma-rays, High energy astrophysics, Positron annihilation, Positrons|
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